1. Field of the Invention
The present invention relates to a cathode ray tube and is directed more particularly to a cathode ray tube in which a light image is observed from its phosphor screen on the side where the electron beam scans.
2. Description of the Prior Art
In a cathode ray tube, the phosphor screen formed on the inner surface of a panel portion of its envelope is impinged with the electron beam emitted from an electron gun located within the neck portion of the envelope to excite the phosphor screen to thereby emit light and hence to produce an image. In a cathode ray tube in which the light image on the phosphor screen is to be observed from the panel side of the envelope, i.e., the side of the glass opposite to that on which the electron beam is impinged, a metal back made of an aluminium layer of a thickness from about 1000 .ANG. to 4000 .ANG. is generally coated on the side of the phosphor screen on which the electron beam is impinged. Therefore, the problem that negative ions accelerated to the phosphor screen by the high voltage provided to the phosphor screen within the envelope impinge directly on the phosphor screen (causing deterioration of its luminance or so-called ion burn) is avoided.
In case of such a cathode ray tube wherein the emitted light image from the phosphor screen is observed from the side of the phosphor screen which is scanned by the electron beam, the aforesaid metal back is not formed on that side of the phosphor screen due to the fact that the light image is derived or observed from that side of the phosphor screen. In this case, since the electron beam directly scans the phosphor screen, the ions which are accelerated impinge on the phosphor screen directly and hence the problem of ion burn is caused.
As methods to avoid the above ion burn there are proposed methods such as to locate a magnet for an ion trap, magnet focus means, and so forth. However, any of such proposed methods result in a construction in which the whole length of the cathode ray tube becomes undesirably long.